The present invention relates to a method for fabricating a semiconductor device that can prevent resist collapse from being caused by resist patterning in a dry etching process.
A known method for fabricating a semiconductor device will be described with reference to the drawings.
FIGS. 3A through 3D show the cross-sectional structures of a semiconductor device in the order corresponding to process steps of the known method for fabricating the same.
First, as shown in FIG. 3A, a thermal oxide film 102 is formed in the upper part of a semiconductor substrate 101 made of silicon. Subsequently, the top of the formed thermal oxide film 102 is coated with a resist film. Thereafter, the resist film is patterned using a lithography method, thereby forming a resist pattern 103.
Next, as shown in FIG. 3B, dry etching is performed on the thermal oxide film 102 by using the resist pattern 103 as a mask. For example, when capacitively coupled plasma etching equipment is employed, etching conditions are as follows: tetrafluorocarbon (CF4) is supplied at a flow rate of 50 ml/min; trifluoromethyl (CHF3) is supplied at a flow rate of 30 ml/min; oxygen (O2) is supplied at a flow rate of 5 ml/min; the gas pressure is 5 Pa; the upper discharge power is 1000 W; and the lower discharge power is 1500 W. Here, the flow rate of each gas is the one in a standard state, i.e., under 0° C. and 1 atm.
In recent years, the processing precision of semiconductor devices has become finer, and thus smaller pattern sizes have also been required for resist patterns 103 used as masks for patterning a film to be etched. Therefore, the physical strength of the resist patterns 103 has become smaller (see, for example, International Publication Number WO98/32162 pamphlet).
In addition, even when a semiconductor device becomes finer, the thickness of a film to be etched hardly changes. This disables the thickness of a resist pattern 103 to become smaller, because it is necessary that the selectivity to the resist at dry etching is ensured. Thus, the value of the aspect ratio (the height to the line width) of the resist pattern 103 at pattern formation has been larger.
On the other hand, a dry etching process allows the resist pattern 103 to be etched not only in the vertical direction but also in the parallel direction with respect to the principal surface of the semiconductor substrate 101. This results in the line width of the resist pattern 103 becoming smaller during etching. Furthermore, an influence of heat and ultraviolet radiation coming from plasma used for dry etching causes stresses associated with heat stresses and degradation in the resist pattern 103. Consequently, as the processing precision becomes finer, the upper part of the resist pattern 103 collapses from the insufficient strength of the resist pattern 103 as shown in FIG. 3B, i.e., a so-called resist collapse 103a occurs. The resist pattern 103 in which the resist collapse 103a occurs serves as an etching mask as it is, so that a part of the thermal oxide film 102 below the resist collapse 103a is prevented from being etched. Consequently, a pattern abnormality 102a is formed in the thermal oxide film 102 as shown in FIG. 3C.
Therefore, as shown in FIG. 3D, even when the resist pattern 103 is removed by performing ashing and cleaning processes, the pattern abnormality 102a remains in the thermal oxide film 102 as it is.
As described above, the known method for fabricating a semiconductor device has the problem that a resist collapse 103a occurs in a resist pattern 103 at the etching of a film to be processed.